Electrochemical devices deliver energy via spontaneous electron migration from one electrode to the other through an external circuit, driven either by Faradaic reactions, e.g. in batteries, or by polarization, e.g. in electrical double layer (EDL) capacitors (EDLCs). In a battery, solid-state Faradaic processes often lead to poor cycling reversibility and limited power performance. EDLCs, or supercapacitors, are based upon the EDL phenomenon at the interface between a polarized electrode and a liquid electrolyte. One EDLC is composed of two EDLs linked in series by an electrolyte bridge. Operation of EDLCs involves neither inter-electrode mass transfer nor solid-state ion diffusion, which leads to long cycling life and high-power. Significant progress has been made to EDLCs in terms of power densities and physical flexibility. Unfortunately, the low energy densities of EDLCs, typically <5 W·h/kg, seriously limit applications. To increase energy density, redox-active oxides, e.g. RuO2 or MnO2, have been added to electrodes to provide so-called “pseudo-capacitance” that is associated surface Faradaic redox chemistry. These devices exhibit compromised power performance and cycle lifetime, compared to EDLCs. Recently, incorporating solvated redox-active species into electrolytes has been reported to improve charge storage. One advantage of using soluble redox species is that the charge/discharge processes do not involve solid-state reactions or solid-state diffusion. A capacitor using KI and VOSO4 solutions separated by a Nafion membrane into two compartments of a cell, as catholyte and anolyte, respectively has been reported. Enhanced energy density was observed. Nevertheless, the expensive Nafion membrane limits the practical application of this design. The use of the ion-selective separator reflects the challenge of controlling the self-discharge reaction between catholyte and anolyte.
What is needed is an energy storage device capable of battery-level energy density, capacitor-level durability and power density in one device.